The invention relates to a sensor sheath for a thermal air flow rate sensor that is intended for disposition in a pipeline through which a fluid flows, and to a sensor device with a sensor that is surrounded by a sensor sheath of this kind.
From German Patent DE 44 07 209 C2, a measuring device is known that serves to measure the mass, or flow rate, of a flowing medium, in particular the aspirated air mass of internal combustion engines. This measuring device is embodied in plate form and has a thermal air flow rate meter, for instance with a hot film resistor as in the HFM5 made by Robert Bosch GmbH. The measuring device is located in a cylindrical aspiration line and is oriented in such a way that the measuring element is oriented parallel to the flow of the aspirated air, so that the flow approaching it is optimal.
This kind of measuring device with a hot film resistor, however, is used not only in motor vehicles for controlling the load-dependent mixture ratio of fuel and air but is also often used in process technology in general. The fluid media employed there are sometimes readily combustible or even explosive substances.
By the operation of a thermal air flow rate meter, the flowing medium can be heated locally in such a way that its temperature may reach the ignition temperature. As a consequence, a flame front created after the ignition of the flowing medium would spread in the form of a fire. Conceivably then, in the worst case, the ignition would prove explosive, causing the destruction of the measuring device, its surroundings, and possibly the entire process technology plant.
The sensor sheath has the advantage over the prior art that even if the fluid at the sensor ignites, the flame front cannot penetrate the wire grid, and so the fluid located outside the wire grid cannot ignite. Nevertheless, the unimpeded inflow of fluid to the sensor is assured by the wire grid, and so the flow conditions at the point where the sensor is located are not changed, and the measurement is not adulterated.
The wire grid acts to avert the propagation of ignition in two ways. First, the temperature of the fluid, even if it ignites at the sensor, is lowered outside the wire grid because of the thermal capacity of the wire grid in such a way that the temperature of the fluid outside the wire grid remains below the ignition temperature of the fluid. In other words, a thermal sink exists. Second, the wire grid acts as a Faraday cage, so that the throughput of electromagnetic waves, which a flame front can also be considered to be, is reduced markedly.
It is especially advantageous if the wires of the wire grid are arranged on the one hand parallel to the flow direction of the fluid and on the other orthogonally to this flow direction. The parallelogram-shaped mesh formed by the wires of the wire grid are then embodied as rectangles. It is also especially expedient to embody the individual wires in the form of ovals, so that the wires form a wire grid that in cross section is oval parallel and/or perpendicular to the flow direction of the fluid. This shape is advantageous hydraulically and is simple to produce.
A mesh width of less than 1.2 mm, preferably less than 0.8 mm, and in particular less than 0.6 mm is preferred, since then the shielding actions of the wire grid become fully operative, and nevertheless an unhindered flow of fluid through the wire grid to the sensor is possible.
The opening through which the sensor can be introduced into the sheath of the wire grid is preferably precisely large enough that the sensor device fits through the opening without problems; the spacing between the carrier body of the sensor and the edge of the opening is preferably less than the mesh width of the wire grid. This prevents the sheath from having an open surface region in which the aforementioned shielding actions do not exist. In this way, ignition of the fluid is prevented from spreading from the interior of the wire grid to the exterior in the peripheral region of the opening, which under some circumstances could lead to an explosion.
The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of a preferred embodiment taken in conjunction with the drawing.